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Graphene is considered to be a promising candidate to replace indium tin oxide (ITO) as transparent conductive electrodes in optoelectronics applications. We use graphene films as transparent electrodes for THz applications such as phase shifters, reflector, and intensity modulators, leading to a low voltage operation and very small insertion loss. First, we experimentally demonstrate the excellent performance of THz modulators based on graphene/ionic liquid/graphene sandwich structures. The modulation covers a broadband frequency range from 0.1 to 2.5 THz with the modulation depth of up to 99% by applying a small gate voltage of 3 V. To our knowledge, this is the highest modulation ratio from graphene based THz devices to date. We also report a highly efficient tunable THz reflector in graphene. By applying a small gate voltage (up to ± 3 V), the reflectance of graphene is modulated from a minimum of 0.79% to a maximum of 33.4% using graphene/ionic liquid structures at room temperature, and the reflection tuning is uniform within a wide spectral range (0.1 – 1.5 THz). In addition, we design and fabricate terahertz phase shifters based on thin liquid crystal cells sandwiched by two graphene layers. A maximum 10.8 degree phase shift is obtained with 5 V voltage. The proposed phase shifters are fully electrical controllable, continuous tunable, and require very low DC voltages for operation. Finally, we show a high performance THz emitter based on ferromagnetic/nonmagnetic heterostructures. Our THz emitter based on nonmagnetic (NM) and ferromagnetic (FM) heterostructures has a peak intensity exceeding 500 μm thick ZnTe crystals (standard THz emitters). We have also fabricated the devices on flexible substrates with a great performance, and demonstrated that the devices can be driven by low power lasers.

In this paper we present results of photoluminescence (PL), photoluminescence excitation (PLE), and time resolved PL spectroscopy of the 4I13/2 → 4I15/2 transition in Er optical centers in GaN epilayers grown by metal-organic chemical vapor deposition. Under resonance excitation via the higher-lying inner 4f shell transitions and band-to-band excitation of the semiconductor host, the PL and PLE spectra reveal an existence of two types of Er optical centers from isolated and the defect-related Er centers in GaN epilayers. These centers have different PL spectra, local defect environments, decay dynamics, and excitation cross-sections. The isolated Er optical center, which can be excited by either excitation mechanism, has the same decay dynamics, but possesses a much higher cross-section under band-to-band excitation. In contrast, the defect-related Er center can only be observed through band-to-band excitation but has the largest crosssection. Our results indicate pathways for efficient optical excitation of Er-doped GaN semiconductors.

Details are presented concerning the growth of GaN and InGaN epilayers by metal-organic chemical vapor deposition and their in situ doping with erbium (Er). Structural properties of the Er-doped epilayers were examined using X-ray diffraction, atomic force microscopy, and secondary ion mass spectrometry. The photoluminescence characteristics of these films were measured and strain-induced effects were identified. Multilayer structures were processed into prototype light-emitting diodes that yielded an infrared emission at 1540 nm. Magnetic measurements of the Er-doped films were made at room temperature and hysteretic behavior was observed. Resonant excitation spectroscopy combined with an applied magnetic field, showed that the Er ions enter into a defect-related complex with critical impact on the optical and magnetic properties.

Doping of GaN and InGaN epilayers with Mn and FE has been achieved by metal organic chemical vapor deposition (MOCVD) and solid-state diffusion. Hysteretic behavior was observed at room temperature and structural measurements, X-ray diffraction, and atomic force microscopy gave no evidence of second phases in the transition metal-doped epilayers. Multilayer structures grown by MOCVD were used to determine the dependence of magnetic ordering in GaMnN epilayers upon the availability of free holes. Based on these results a prototype spin device was designed and fabricated in which holes, adjacent to a GaMnN layer, could be modulated by an applied electric field. The device demonstrated that the transition from paramagnetic to ferromagnetic behavior could be switched on and off electrically at room temperature.

The deposition of amorphous indium zinc oxide (IZO) thin films on glass substrates with n-type carrier concentrations between 1014 and 3 × 1020 cm−3 by sputtering from single targets near room temperature was investigated as a function of power and process pressure. The resistivity of the films with In/Zn of ∼0.7 could be controlled between 5 × 10−3 and 104 Ω cm by varying the power during deposition. The corresponding electron mobilities were 4–18 cm2 V−1 s−1.The surface root-mean-square roughness was

Spin based electronics is an emerging field where the spin of electrons in addition to the charge of electrons is manipulated and different functionalities in devices are achieved. Developments of new materials, that are ferromagnetic at room temperature such as GaMnN, will allow us to create spin- polarized electrons which are prerequisite for progress in this field. Theory suggests that GaN when alloyed with transition metals such as Mn at low concentrations (5 % Mn) will be ferromagnetic. The growth and the origin of the magnetic property in this crystal are still under investigation. We demonstrate the growth and characterization of GaMnN films on (0001) sapphire substrates by MOCVD. We have found that the magnetization in these films correlate with the position of the Fermi level in the crystal. Moreover, changing the position of the Fermi level by co-doping changes the magnetization observed in these films . Our experiments reveal a possible mechanism of the observed magnetic property in this material system. We have also found that charge transfer across ferromagnetic and non magnetic interfaces resulted in changes in the magnetic properties

Spin-dependent phenomena in ZnO may lead to devices with new or enhanced functionality, such as polarized solid-state light sources and sensitive biological and chemical sensors. In this paper, we review the experimental results on transition metal doping of ZnO and show that the material can be made with a single phase at high levels of Co incorporation (~15 at.%) and exhibits the anomalous Hall effect. ZnO is expected to be one of the most promising materials for room-temperature polarized light emission; but to date, we have been unable to detect the optical spin polarization in ZnO. The short spin relaxation time observed likely results from the Rashba effect. Possible solutions involve either cubic phase ZnO or the use of additional stressor layers to create a larger spin splitting in order to get a polarized light emission from these structures or to look at alternative semiconductors and fresh device approaches

Single-phase GaGdN and GaGdN:Si films were grown on sapphire substrates by gas source molecular beam epitaxy using solid Gd, Ga, and Si sources and active nitrogen derived from a RF nitrogen plasma source. The undoped films were highly resistive films but became conductive with the addition of Si. Superconducting quantum interference device magnetometry indicated room-temperature ferromagnetism in both types of materials. Structural defects had a strong influence on the magnetic ordering of the material, as seen in a drastic reduction of magnetic moment with degrading crystalline quality. Magnetization of the codoped film increased with Si content, reaching levels higher than that of the undoped material. The Gd-doped AlN films grown in a similar fashion also displayed Curie temperatures above room temperature.

Silicon photonics is rapidly growing and a number of Si-based active and passive components have recently been demonstrated. We demonstrate new functionality of Er-doped silicon: a memory effect in electroluminescence. This finding opens a prospect of necessary, and thus far not available, component for Si optoelectronics-a fully complimentary metal-oxide-semiconductor-compatible electro-optical converter with a memory function, operating in the technologically important 1.5-mum band. When developed and optimized, prospect applications could include optical intraand inter-chip connectors and volatile flash memory elements

The properties of semi-insulating GaN films doped with Fe are reported. The 300 K sheet resistivity of the films is 2 × 1010 Ω/square with an activation energy of the dark conductivity of 0.5 eV. The Fermi level is also pinned at EC–0.5 eV. The concentration of the 0.5 eV traps in the Fe doped portion of the films was 3 × 1016 cm–3. Also present is a high concentration of deeper electron traps with the level near EC–0.9 eV and of hole traps with level near EV+0.9 eV. Intra-center transitions of the Fe3+ center are observed in the photoluminescence spectra. The stability of the films were studied after rapid thermal annealing (RTA) at temperatures 750–1050 °C and furnace annealing in hydrogen at temperatures up to 850 °C. The Fe is distributed nonuniformly, with a minimum near 0.5–1 µm from the surface. RTA at 850 °C leads to roughness of the surface and decreases of the sheet resistivity and the cathodoluminescence intensity. The density of deep traps also greatly decreases. The effect becomes much more pronounced for furnace annealing in hydrogen for times on the order of 15 minutes and 850 °C is the highest practicable under these conditions without destroying the surface morphology. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Electrical and optical properties of p-AlGaN films grown by metalorganic chemical vapour deposition and implanted with high doses (3 × 1016 cm−2) of 250 keV Mn and Cr ions are reported. Schottky diodes prepared on such implanted films after annealing at 700 °C show a strongly increased reverse leakage current and increased series resistance in the forward direction due to residual radiation damage related defects. The density of these defects was considerably higher for the Mn-implanted diodes which correlated with the higher expected radiation damage due to higher ion mass of Mn compared with Cr. Deep level measurements revealed the formation of hole traps with levels near Ev + 0.4 eV and Ev + 1 eV. Optical absorption measurements indicated the presence of two major defect bands at about 1.5–1.7 eV and 2.25–2.35 eV. The results are compared with earlier measurements on Cr- and Mn-implanted p-GaN films and on proton-implanted p-AlGaN films.

Fundamental electrical and optical properties of strained wurtzite InGaN/GaN-based quantum-well light-emitting diodes are calculated based on the Rashba–Sheka–Pikus Hamiltonian in the vicinity of the Γ point. It is found that the strain and the strain-induced piezoelectric field significantly alter the subband structure and determines the output intensity of the nitride quantum well light emitting diodes. For the case with high In composition (≳0.2), the calculation also supports the possibility of strain relaxation in the quantum well. Coupled with an optimized set of parameters, our theoretical model provides an excellent agreement with the available experimental data over a wide range of In composition (0–0.5).

The temperature dependent behavior of continuous-wave and time-resolved photoluminescence of Eu-doped GaN in the visible region is measured for both the 5D0→7F2 and 5D0→7F3 transitions. The radiative decay of these transitions, following pulsed laser excitation of the GaN host, is monitored by a grating spectrometer and photomultiplier tube detector system. In addition to these two radiative energy transfer pathways within Eu3+, the data reveal two nonradiative energy transfer paths between Eu3+ and the host GaN. Decay constants for the relaxation processes are extracted from the data using a numerically solved rate equation model. Although the dominant radiative relaxation processes decayed with a temperature insensitive decay constant of 166 μs, a prominent role for nonradiative transfer between Eu3+ and impurities within the GaN host was deduced above 180 K.

We investigated different Er3+ defect sites found in Er-doped GaN layers by site-selective combined excitation-emission spectroscopy and studied the role of these sites in different direct and multistep excitation schemes. The layers were grown by molecular beam epitaxy and were 200 nm thick. Two majority sites were found along with several minority sites. The sites strongly differ in excitation and energy transfer efficiencies as well as branching ratios during relaxation. For this reason, relative emission intensities from these sites depend strongly on emission and excitation. The sites were identified for several transitions and a comprehensive list of energy levels has been compiled. One of the minority sites appears strongly under ultraviolet excitation above the GaN band gap suggesting that this site is an excellent trap for excitation energy of electron-hole pairs.

Electrical and optical properties and the spectra of deep hole traps in p-GaN films implanted with high doses (3 × 1016 cm−3) of Co,Mn, Fe and Cr and annealed at 700 °C are reported. The dominant deep traps generated by this implantation are of the same type as observed in similar films heavily implanted with protons. The magnitude of the changes in the conductance and transmission of the GaN correlates with the atomic mass of the transition metal ions and the density of primary radiation defects. For fabrication of spintronic devices such as spin-LEDs using implantation of the TM species into the top p-GaN contact layer, the best results should be expected using Cr implantation since this produces both room temperature ferromagnetism and the smallest reduction in conductivity of the p-GaN.

Recent results on achieving ferromagnetism in transition-metal-doped GaN, A1N and related materials are discussed. The field of semiconductor spintronics seeks to exploit the spin of charge carrier ...

Optical absorption and microcathodoluminescence (MCL) bands were studied in heavily Cr and Fe implanted ZnO bulk crystals annealed at 700°C. The samples showed at room temperature hysteresis. The implanted regions remained n-type and optical absorption bands near 1.8 eV and the MCL bands near 2.8 eV were found to be associated with the transition metal (TM) ions. These bands are tentatively attributed to internal transitions between the levels of the substitutional TM ions. In addition to those bands we also observed strong absorption bands at 0.5 and 2.4 eV and MCL bands near 1.9 and 2.3 eV. All these bands are related to radiation damage defects.

Two different plasma chemistries for etching ZnO were examined. Both Cl2/Ar and CH4/H2/Ar produced etch rates which increased linearly with rf power, reaching values of � 1200 A for Cl2/Ar and � 3000 A for CH4/H2/Ar. The evolution of surface morphology, surface composition, and PL intensity as a function of energy during etching were monitored. The effect of H in ZnO was studied using direct implantation at doses of 10 15 –10 16 cm � 2 , followed by annealing at 500–700 C. The hydrogen shows significant outdiffusion at 500 C and is below the detection limits of SIMS after 700 C anneals. SEM of the etched features showed anisotropic sidewalls, indicative of an ion-driven etch mechanism.

Hydrogen is readily incorporated into bulk, single-crystal ZnO during exposure to plasmas at moderate (100–300°C) temperatures. Incorporation depths of >25 μm were obtained in 0.5 h at 300°C, producing a diffusivity of ∼8 × 10−10 cm2/V s at this temperature. The activation energy for diffusion is 0.17 ± 0.12 eV, indicating an interstitial mechanism. Subsequent annealing at 500–600 °C is sufficient to evolve all of the hydrogen out of the ZnO, at least to the sensitivity of Secondary Ion Mass Spectrometry (

Both ion implantation and epitaxial crystal growth provide convenient methods of introducing transition metals such as Mn,Cr,Fe,Ni and Co into GaN, GaP, SiC and ZnO for creating dilute magnetic semiconductors exhibiting room temperature ferromagnetism. In this paper we review progress in wide band gap ferromagnetic semiconductors and the role of defects and doping on the resulting magnetic properties.

The effects of hydrogen on the electrical and optical properties of p-GaN were investigated. Hydrogen is readily incorporated into the material at temperatures of 250–350 °C, which is consistent with the low activation energy for diffusion reported by Seager et al. [J. Appl. Phys. 92, 7246 (2002)] in GaN p-n junctions. From comparison with the results of earlier experiments, hydrogen diffusivity appears to be a strong function of the hydrogen concentration incorporated into the material during growth. More than an order of magnitude decrease in hole concentration was observed after the hydrogen plasma treatment and from the measurements of the temperature dependence of conductivity. This is the result of hydrogen passivation of acceptors rather than of increased compensation by donor centers. Hydrogen treatment was also shown to lead to a strong suppression of 0.3 eV and 0.6 eV traps and to a strong increase in the magnitude of the photocurrent which are the results of passivation of deep-level defects by...

Electrical and optical properties of undoped n-ZnO crystals implanted with 50 keV protons with doses from 5×1013 to 5×1015 cm−2 are reported. Proton implantation leads to a decrease of the carrier concentration in the near-surface region, but at the end of the proton range shallow donors are observed whose concentration tracks the implant dose and that we attribute to hydrogen donors. Three deep electron traps with apparent activation energies of 0.55, 0.75, and 0.9 eV are introduced by proton implantation. The 0.9 eV traps have been observed through the increased thermal stability of the Schottky diodes prepared on heavily implanted n-ZnO compared to unimplanted or lightly implanted samples. In addition, hole traps located 0.16 eV above the valence band edge were introduced by implantation. Proton implantation also led to the formation of persistent-photocapacitance-active defects and to considerable decrease in the intensity of the band edge luminescence and in the value of the photocurrent of the Au/n-...

The electrical properties, deep level spectra, and microcathodoluminescence (MCL) spectra of p-GaN films implanted with 100 keV protons are reported. Measurable decreases of the MCL intensity began for doses as low as 1012 cm−2, while measurable decreases of the hole concentration started for doses of 1013 cm−2. The main deep traps introduced by protons had activation energies of 0.3, 0.6, and 0.9 eV. The 0.3 and 0.9 eV traps are located in the lower half of the band gap and determine the temperature dependence of the forward current in heavily proton implanted Schottky diodes.

Implantation of Mn or Cr at doses of 3–5 × 1016 cm−2 into Si-doped Al0.24Ga0.76P epilayers on GaP substrates produced ferromagnetic ordering at temperatures up to 300 K. The results were similar to those obtained previously in p-type AlGaP(C), indicating that both electron and hole-doped AlGaP can exhibit ferromagnetism. In addition, the AlGaP results are similar to those for GaP so the magnitude of the bandgap was not the main parameter influencing the Curie temperature, in contradiction to predictions from some mean-field theories. Second phases were not observed by X-ray diffraction and were not responsible for the ferromagnetism.

Electrical properties, room temperature optical absorption bands and 300 and 90 K microcathodoluminescence (MCL) bands were studied in heavily Mn and Co implanted ZnO samples annealed at 700 °C. Samples processed under the same conditions have previously shown ferromagnetic behavior. The Mn and Co implanted and annealed regions remain n-type and have a higher electron concentration than in control samples. Therefore, the spin alignment in ZnMnO and ZnCoO is not mediated by the classical mechanism involving free holes. Optical absorption bands near 1.9 eV (Co) and 2 eV (Mn) and the MCL bands near 1.84 eV (Co) and 1.89 eV (Mn) are found to be associated with transition metal (TM) ions. These bands are tentatively attributed to internal transitions between the levels of the substitutional TM ions. In addition to those TM ions related bands we also observed strong absorption bands at 0.75, 1.4, 2.35 and 2.5 eV and MCL bands near 2.3 and 2.5 eV. All these bands are related to radiation damage defects.

The effects of hydrogen plasma treatment on high-quality bulk n-ZnO crystals were studied. It is shown that after plasma exposure at 200 °C for 0.5 h the hydrogen penetrates into the material down to about 20 μm and shows concentrations close to 1017 cm−3 in that region. The incorporation of this hydrogen coincides with an increase in the shallow donor concentration to about the same level as the concentration of hydrogen. In contrast to that in most other semiconductor materials, hydrogen plasma treatment of ZnO is shown to increase the concentration of the already existing electron and hole traps and to introduce electron traps near 0.55 eV, earlier observed in proton irradiated samples. The effect is at least partially due to the surface damage caused by plasma exposure. Despite this increase in the density of deep traps, the luminescence intensity in the near band-edge region is shown to increase down to the depth corresponding to the hydrogen penetration depth in the material.

Deep levels introduced into n-GaN films by Fe and Cr implantation have been studied by means of optical absorption and microcathodoluminescence spectroscopy measurements and by deep level transient spectroscopy, admittance spectroscopy, and capacitance-voltage profiling. The results are compared with previous measurements on Mn and Co implanted GaN. It is shown that the acceptor levels of substitutional Mn, Co, Fe, and Cr in n-GaN are located, respectively, near Ev+1.6 eV, Ev+1.7 eV, Ev+1.8 eV, and Ev+2 eV, the trend being similar to that observed in GaAs, GaP, and InP. The Fermi level in the implanted region is pinned near deep electron traps at Ec−0.5 eV that are tentatively attributed to complexes between substitutional transition metal ions and native defects such as nitrogen vacancies. It is shown that for all implanted species after 700 °C annealing a damaged region with relatively high resistivity is formed down to the depth of about 1 μm much, exceeding the projected range of implanted ions. This ...

We present experimental evidence of an optical memory effect in crystalline silicon doped with Er3+ ions. It is observed at low temperature using two-color experiments in the visible and the mid-infrared (with a free-electron laser). Based on the physical mechanism governing the effect, possibilities for improvement of thermal stability and increase of archival time are discussed. An all-optical all-silicon memory element for use in photonic circuits is proposed.

The green (537 and 558 nm) and near infrared (1.54 μm) photoluminescence (PL) spectra of Er-doped GaN thin films have been investigated as a function of temperature, excitation wavelength, and pump intensity. Thermal quenching measurements showed that the integrated green Er 3+ PL intensity ( 4 S 3/2 / 2 H 11/2 → 4 I 15/2 ) remained nearly constant up to 150 K, but decreased at higher temperatures due to a less efficient Er 3+ excitation. The integrated infrared Er 3+ PL intensity ( 4 I 13/2 → 4 I 15/2 ) was found to be temperature-independent up to 250 K, but decreased slightly at higher temperatures due to the onset of non-radiative decay. Pump intensity PL studies revealed that the above-gap excitation cross-section is more than two orders of magnitude greater than the below-gap excitation cross-section. Within a simplified three-level model, the above-gap excitation cross-section was estimated to be ∼10 −16 cm 2 . This result indicates that Er 3+ ions can be excited efficiently through carrier-mediated processes in a forward-biased GaN:Er light emitting device.

High concentrations ( � /10 21 cm � 3 ) of Ni were introduced into GaN and SiC by ion implantation at 350 8C. On subsequent annealing at 700 8C, there was more residual lattice damage in GaN compared to SiC. Both materials showed ferromagnetism with transition temperatures below 50 K. No secondary phases could be detected by transmission electron microscopy (TEM) or selected area diffraction in either GaN or SiC. The direct implantation process appears useful for studying ion/substrate combinations for potential spintronic applications. # 2002 Elsevier Science B.V. All rights reserved.

Structural and magnetic characteristics of Ni-implanted GaP were measured for doses in the range 3/5/10 16 cm 2 . After subsequent annealing at 700 8C, transmission electron microscopy (TEM) showed residual lattice damage which was more significant in highly carbon-doped epi layers as compared to bulk GaP substrates. The magnetization measurements showed two different contributions, one present at 5/75 K and the other below � /225 K. No secondary phase formation was detected in either type of GaP. # 2002 Elsevier Science B.V. All rights reserved.

Direct implantation of Fe, Ni or Mn at doses of 3–5×1016 cm−2 into p-type 6H-SiC substrates was carried out at a sample temperature of ∼350 °C. Subsequent annealing was performed at 700–1000 °C for 5 mins. Residual damage in the form of end-of-range defects and dislocation loops in the region from the surface to a depth of ∼0.20 μm were examined by transmission electron microscopy. To the sensitivity of both x-ray diffraction and selected area diffraction pattern analysis, no secondary phases could be detected. Signatures of ferromagnetism were observed in all the highest dose samples, with apparent Curie temperatures of 50 K (Ni), 250 K (Mn), and 270 K (Fe).

High concentrations (0.1-5 at.%) of Mn or Fe were introduced into the near-surface region (≤2000 A) of 6H-SiC substrates by direct implantation at ˜300°C. After annealing at temperatures up to 1000°C, the structural properties were examined by transmission electron microscopy (TEM) and selected-area diffraction pattern (SADP) analysis. The magnetic properties were examined by SQUID magnetometry. While the Mn-implanted samples were paramagnetic over the entire dose range investigated, the Fe-implanted material displayed a ferromagnetic contribution present at >175 K for the highest dose conditions. No secondary phases were detected, at least not to the sensitivity of TEM or SADP.

The magnetization of p-GaN or p-AlGaN/GaN superlattices was measured after implantation with high doses (3–5×1016 cm−2) of Mn, Fe, or Ni and subsequent annealing at 700–1000 °C. The samples showed ferromagnetic contributions below temperatures ranging from 190–250 K for Mn to 45–185 K for Ni and 80–250 K for Fe. The use of superlattices to enhance the hole concentration did not produce any change in ferromagnetic ordering temperature. No secondary phase formation was observed by x-ray diffraction, transmission electron microscopy, or selected area diffraction pattern analysis for the doses we employed.

In p-GaN implanted with Mn (3 × 1016cm-2 at 250 keV), the material after annealing shows ferromagnetic properties below 250 K. Cross-sectional transmission electron microscopy (TEM) revealed the presence of platelet structures with hexagonal symmetry. These regions are most likely GaxMn1-xN, which produce the ferromagnetic contribution to the magnetization. In p-GaN implanted with Fe, the material after annealing showed ferromagnetic properties at temperatures that were dependent on the Fe dose, but were below 200 K in all cases. In these samples, TEM and diffraction analysis did not reveal any secondary phase formation. The results for the Fe implantation are similar to those reported for Fe doping during epitaxial growth of GaN.

The structural and magnetic properties of p-GaN implanted with high doses of Mn + or Fe + (0.1-5 at%) and subsequently annealed at 700-1000 °C were examined by transmission electron microscopy, selected-area diffraction patterns, X-ray diffraction and SQUID magnetometry. The implanted samples showed paramagnetic behavior on a large diamagnetic background signal for implantation doses below 3 at% Mn or Fe. At higher doses the samples showed signatures of ferromagnetism with Curie temperatures

We have conducted a systematic study of the bandedge and infrared luminescence properties of Er-implanted GaN thin films. The GaN films, grown by metalorganic chemical vapor deposition, were co-implanted with Er and O ions. After implantation, the implanted samples were furnace annealed at temperatures up to 1100°C. Following annealing, the samples were examined for both bandedge luminescence and for infrared luminescence near 1540 nm. It was observed that the bandedge photoluminescence (PL) was significantly reduced in the as-implanted samples. In addition, there was no detectable PL signal near 1540 nm, with either above-bandgap or below-bandgap excitation. Only after annealing at temperatures above 900°C did both the bandedge luminescence and the 1540 nm luminescence become well defined. An optical transition at 3.28 eV was also observed, apparently induced through Er+O implantation. While annealing at higher temperatures resulted in a decrease in the 1540 nm luminescence, emission intensities from the bandedge and the defect level both increased.

Standard variable angle spectroscopic ellipsometry (VASE) has been employed to study the ordinary optical dielectric response of hexagonal gallium nitride (GaN) thin films—an important material for blue and ultraviolet light emitting device applications. The GaN films were grown by molecular beam epitaxy on c-plane sapphire substrates (α-Al2O3). Room temperature isotropic and anisotropic mode VASE measurements were made at angles of incidence between of 20° and 80°. Evidence of anisotropy was observed from the anisotropic mode measurements, reflecting the nature of wurtzite crystal structure of GaN. The sizable off-diagonal elements (Aps and Asp) of the Jones matrix indicate that the optical axis 〈c〉 of the c-plane sample are slightly off from the surface normal due to a small miscut of substrates. VASE data simulations by isotropic and anisotropic models indicate that the anisotropic effect on both diagonal and off-diagonal elements of the Jones matrix can be minimized to a negligible level at small angl...

Semiconductors doped with rare earth atoms have been studied for more than a decade because of the potential of using them to develop compact and efficient electroluminescence (EL) devices. Trivalent erbium ions (Er 3+ ) are of special interest because they exhibit atomic-like transitions centered at 1540 nm, which corresponds to the low-loss window of silica-based optical fibers. While EL devices, based on Er-doped Si and GaAs materials, have been fabricated, their efficiency remains too low for practical applications. Several years ago an important observation was made that there was less detrimental temperature quenching of Er luminescence intensity for larger bandgap host materials. Therefore, Er-doping of wide gap semiconductors, such as the III–V nitrides, appears to be a promising approach to overcoming the thermal quenching of Er luminescence found in Si and GaAs. In particular, GaN epilayers doped with Er ions have shown a highly reduced thermal quenching of the intensity of the Er luminescence from cryogenic to elevated temperatures. The remarkable thermal stability of the light emission may be due to the large energy bandgap of the material, as well as to the optical inactivity of the material defects in the GaN films. In this paper, recent data concerning the luminescence characteristics of Er-doped GaN thin films are presented. Two different methods have been used for Er-doping of the GaN films: ion implantation and in situ doping during epitaxial growth. Both methods have proven successful for incorporation and optical activation of Er 3+ ions. Infrared photoluminescence spectra, centered at 1540 nm, have been measured for various Er-doped III–N films. Considerably different emission spectra, with different thermal quenching characteristics, have been observed, depending upon the wavelength of the optical pump and the Er-doping method. Defect-related absorption centers permit excitation of the Er ions using below-bandgap optical sources. Elemental impurities, such as O and C, in the thin films have also been shown to influence the emission spectra and to lead to different optical characteristics.

Erbium (Er) doped semiconductors are of interest for light-emitting device applications operating at around 1.55 μm and for the potential integration with other semiconductor devices. However, the optical emission of Er3+ ions in semiconductors has not been as efficient as in dielectric materials, particularly at room temperature. This may be because ionic bonds, which are characteristic of dielectrics, are better suited for forming the required Er3+ energy levels than are covalent bonds, which are present in most III-V semiconductors. In this paper, we report 1.55 μm emission from an Er-doped GaN LED. We also discuss the effect of the measurement temperature on the emission spectrum as well as the effect of sample annealing on the emission spectrum.

Donor (S, Se and Te) and acceptor (Mg, Be and C) dopants have been implanted into GaN at doses of 3-5x1014 cm-2 and annealed at temperatures up to 1450 *C. No redistribution of any of the elements is detectable by Secondary Ion Mass Spectrometry, except for Be, which displays an apparent damage-assisted diffusion at 900 "C. At higher temperatures there is no further movement of the Be, suggesting that the point defect flux that assists motion at lower temperatures has been annealed. Effective diffusivities are

Phonon modes in column-III--nitride ternary semiconductors are investigated theoretically within the modified random-element isodisplacement model. It is found that ${A}_{1}$ and ${E}_{1}$ branches of optical phonons in wurtzite ${\mathrm{Ga}}_{x}{\mathrm{Al}}_{1\ensuremath{-}x}\mathrm{N}$ and ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}$ exhibit one-mode behavior. This result is explained by the fact that atomic mass of nitrogen is much smaller than those of other atoms.

AlN(Er) doped with Er during metal organic molecular beam epitaxy has been plasma hydrogenated in situ at 200–250 °C using an electron cyclotron resonance source. By isotopic substitution of 2H for 1H, we have found from secondary ion mass spectrometry profiling that a 30 min hydrogenation treatment can incorporate ∼2×1019 cm−3 deuterium atoms to depths ⩾1 μm. The intensity of the 1.54 μm Er3+ luminescence is increased by a factor of ∼5 by the 200 °C hydrogenation, and this effect is thermally stable to 300 °C, indicating a binding energy of >1.5 eV for hydrogen at defects in the AlN. These defects would normally either be recombination centers or provide an alternative de-excitation path for the Er. We have previously found that AlN provides the best resistance to thermal quenching of Er luminescence of any semiconductor due to its wide bandgap. Together, these results suggest that AlN(Er) may be a promising material for optical control of devices such as light-triggered SiC or GaN thyristors for power s...

GaN, AIGaN and InGaN films have been grown by molecular beam epitaxy (MBE) using RF plasma sources for the generation of active nitrogen. These films have been deposited homoepitaxially onto GaN/SiC substrates and hetero-epitaxially onto LiGaO 2 substrates. LiGaO 2 is an ordered and closely-lattice-matched orthorhombic variant of the wurtzite crystal structure of GaN. A low-temperature AIN buffer layer is necessary in order to nucleate GaN on LiGaO2. Thick GaN and AIGaN layers may then be grown once deposition is initiated. InGaN has been grown by MBE at mole fractions of up to 20% as a quantum well between GaN cladding layers. The indium containing structures were deposited onto GaN/SiC substrates to focus the development effort on the InGaN growth process rather than on heteroepitaxial nucleation. A modulated beam technique, with alternating short periods of (In, Ga)N and (Ga)N, was used to grow high-quality InGaN. The modulated beam limits the nucleation of metal droplets on the growth surface, which form due to thermodynamic limitations. A narrow PL dominated by band edge luminescence at 421 nm results from this growth technique. Growth of GaN at high temperatures is also reported.

Solid source Er doping of AlN and GaN during growth by plasma-assisted metalorganic molecular beam epitaxy (MOMBE) with an Er effusion source has resulted in AlN : Er and GaN : Er exhibiting room-temperature 1.54 μm photoluminescence (PL). The luminescence detected in the AlN : Er samples was orders of magnitude greater in intensity than that from ion-implanted material and represents the first demonstration of strong emission from rare-earth doped, epitaxial III-nitrides doped during growth. The effects of growth temperature on Er incorporation and segregation behavior have also been examined. PL studies, including room temperature and thermal quenching experiments have been conducted showing a reduction in PL intensity of approximately a factor of two between 15 K and room temperature for strongly luminescent AlN : Er. Preliminary results from GaN : Er indicated that thermal quenching is considerably greater in this material than in AlN : Er.

Several kinds of nitride-based micro-resonators have been fabricated. Firstly, a microdisk laser structure comprising three InGaN GaN quantum wells on a thick AlN buffer has been grown by metal-organic molecular beam epitaxy and fabricated using a combination of non-selective Cl2/CH4/H2/Ar dry etching and selective KOH-based wet etching of the AlN. These structures are of potential use in short wavelength photonic or optoelectronic circuits. In a second structure Er was implanted into a GaN layer to produce strong emission at 1.54 μm. Similar results have been obtained in Er-implanted AlN, and AlN doped during epitaxial growth.